Troponin is a component of thin filaments (along with actin and tropomyosin), and is the protein to which calcium binds to accomplish this regulation. Troponin has three subunits, TnC, TnI, and TnT.

Troponin I is a part of the troponin complex. It binds to actin in thin myofilaments to hold the actin-tropomyosin complex in place Because of which myosin can not bind the actin in relaxed muscle. When Calcium binds to the Troponin C it causes certain confirmational changes which leads to dislocation of troponin I and finally tropomyosin leaves the the binding site for myosin on actin leading to contraction of muscle. The letter I is given due to its inhibitory character.The tissue specific subtypes are: Slow-twitch skeletal muscle isoform troponin I, TNNI1; Fast-twitch skeletal muscle isoform troponin I, TNNI2; Cardiac troponin I, TNNI3.